A 5-year-old girl from Bangalore presents on day 6 of fever with abdominal pain, persistent vomiting, and lethargy. Her parents report she had dengue fever 2 years ago. On examination, she is pale, with a blood pressure of 88/54 mmHg (hypotensive for age), heart rate 128/min, and cool extremities. Liver edge is palpable 3 cm below the costal margin. Laboratory findings: platelet count 35,000/μL, haematocrit 48% (baseline 38%), AST 1,200 IU/L, albumin 2.8 g/dL. What is the most likely diagnosis and the next immediate step?